Pipette

ABSTRACT

A pipette, comprising: a cylinder (1) with liquid passage (3), a plunger (4), and a plunger operating assembly (5) for moving the plunger in the cylinder; the operating assembly comprising a body (6), a threaded rod (7), an operating member (8) cooperating by mediation of threads with the threaded rod, and a power unit (9) for rotating the threaded rod and the operating member relative to each other for moving the plunger by mediation of the threaded rod and the operating member when the threaded rod and the operating member are rotated relative to each other with the aid of the power unit; the thread of the threaded rod (7) or alternatively that of the operating member (8) being non-linear locally over a certain transition range.

BACKGROUND OF THE INVENTION

The present invention concerns a pipette comprising a cylinder whichconstitutes a cylinder volume with liquid passage; a plunger placed tobe reciprocatingly movable in said cylinder volume; and aplunger-operating means for moving the plunger within the cylinder; saidoperating means comprising a body, a threaded rod, an operating memberin cooperation by means of a thread with said threaded rod, and a powermeans for rotating the threaded rod and the operating member relative toeach other; said body being connectable to the cylinder and the threadedrod/operating member combination, to the plunger, for moving the plungerby mediation of the threaded rod and the operating member when thethreaded rod and the operating member are rotated relative to each otherwith the aid of the power means.

The filling of a pipette is based on producing a vacuum in the cylindervolume of the pipette by moving the plunger in the cylinder volume. Atthe same time, liquid which one wants to fill is conducted into thecylinder volume through the tip of the pipette. The true filling volumeof the pipette, i.e., the volume of the liquid quantity that has enteredthe pipette, depends on quite a number of factors, e.g. the displacementvolume of the plunger, ambient air pressure, which in turn is due totemperature and gravity, height of the liquid column inside the pipette,air space inside the pipette, inclination of the pipette (which affectsthe liquid column height), resilience of the plunger sealing, lifting ofthe pipette (which gives rise to a downward inertia force of the liquidcolumn), tip leakage, adhesion forces between tip and liquid, etc. Theair volume of the pipette, in particular, exerts a great effect on thepipetting accuracy.

For the reasons mentioned, the liquid volume, the true filling volume inconnection with pipetting, differs from the corresponding displacementvolume of the plunger by even as much as several per cent. The error ishighest in high precision pipettes which are meant to be used in a wideoperating range, e.g. from 10 μl to 100 μl or 100 to 1000 μl. Thehighest error percentages are encountered at the lowest rated volumes,even up to 2 to 3%.

In practice, when the true filling volume of the pipette is consideredin relation to the displacement volume of the plunger, at the beginningtoo little liquid enters the pipette, compared with the displacementvolume of the plunger, possibly mainly owing to the air volume of thepipette. As operating the pipette is continued, its true filling volumeapproaches relatively the theoretical displacement volume of theplunger. Therefore the relative error is highest expressly at the lowestfilling volume of the pipette.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate the above-mentioneddrawback. It is in particular an object of the invention to provide anovel kind of pipette in which the difference between true fillingvolume and theoretical filling volume can be minimized.

It is in particular an object of the invention to provide a highprecision pipette in the case of which the relative error between itstrue filling volume and its theoretical filling volume can be eliminatedwith substantially higher efficiency than in prior art also at low ratedvolumes.

The invention is based on that the thread of the threaded rod and/or theoperating member is non-linear. The thread is advantageously non-linearin such a way that the non-linearity of the thread causes a non-linear,that is non-uniform, motion of the plunger. Thus, in the pipette of theinvention, when the threaded rod and operating member are rotatedrelative to each other with constant speed, the true movement of theplunger, that is the displacement volume, is not uniformly consistentwith the rotary movement, owing to the non-linearity of the thread and,for instance, the plunger displacement volumes corresponding to a givenangle of rotation of the threaded rod/operating member combination arenot consistent throughout the range of rotation of the threaded rod andoperating member.

The thread of the threaded rod and/or the operating member isadvantageously non-linear so that uniform rotary movement of thethreaded rod and operating member relative to each other, transmitted bythe thread, produces a maximum in the axial movement between thethreaded rod and the operating member, and thereby minimizes themovement, in that region which corresponds to the start of plungermovement from zero volume of the volume, i.e., of the cylinder volume,in the filling direction. Thereby, said maximum in the plunger movementcompensates for the error due to the air volume of the pipette, in theregion close to zero volume of the pipette, in the filling direction.

The non-linearity of the thread is advantageously based on the flankangle of one flank of the thread being constant (α) over part of thethread and constant and greater (α¹) over part of the thread and thesmaller flank angle increasing up to the greater one in a transitionrange. Then, part of the other flank of the thread, i.e., of thesupporting surface, may be removed over part of the thread so that thepitch of the supporting surface changes in the transition range; in thetransition range that part of the thread on which some of the supportingsurface has been removed joins the remaining part of the thread. Thepitch of the thread is then mainly uniform over the whole range of thethread. The non-linearity of the thread is only observable on onesupporting surface of the thread, the inclination (α) of which isdifferent on part of the thread from that on the remainder of the thread(α¹). In the transition range the inclination α of the non-linearsupporting surface changes to become the inclination α¹. The change maybe linear or otherwise regular, or irregular. The pitch of the otherflank of the thread, i.e., of the other supporting surface, may becompletely uniform over the entire range of the thread. Thanks to thesupporting surface formed by the changing angle of the thread's second,non-uniform flank, the rate of movement of the threaded rod and theoperating member relative to each other changes, i.e., has its maximum,expressly at the transition range, to revert to its original value afterthe transition range, at constant rotating speed of the threaded rod andthe operating member. When the transition range, that is, the threadedrod and operating member, are disposed so that the transition rangeaffects the relative rate of movement of the threaded rod and theoperating member in axial direction of the threaded rod exactly fromzero volume of the pipette's cylinder volume in the filling direction,the error between true and theoretical filling volume can be eliminated.It should further be noted that as long as the pitch of the second flankof the thread, that is of the supporting surface, is fully uniform, thetrue and theoretical emptying volumes of the pipette are mutuallyconsistent on the basis of the rotation of threaded rod and operatingmember, as measured when the pipette is being discharged, i.e., in theentire emptying range corresponding substantially linearly, that isuniformly, to the rotation of threaded rod and operating member relativeto each other at uniform rate.

It should be noted that in the pipette of the invention the thread ofthe threaded rod may be non-linear. In another alternative the thread ofthe operating member is non-linear.

Furthermore, in an embodiment of the invention, the pitch of the threadof the threaded rod and/or the operating member, that is the pitch angleof the thread, is uniform and constant over part of the thread anddifferent therefrom, equal to another constant, on the remainder of thethread. This embodiment, too, enables inaccuracy caused by thedifference between true and theoretical filling volume to becompensated. A thread of this kind is inconvenient regardingmanufacturing technique, but its use may be contemplated in some specialapplications.

Thanks to the invention, and when the conditions in which the pipette isfilled are constant, non-uniformity of the thread of the threaded rodand/or the operating member can be applied to eliminate the operatingerrors due to differences between the true filling volume and thetheoretical filling volume (as measured on the basis of the rotations ofthreaded rod and operating member). The thread of the threaded rodand/or the operating member may then be non-uniform throughout,corresponding to the true filling volume of the pipette.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following in detail with the aid ofembodiment examples, referring to the attached drawings, wherein

FIG. 1 presents in a schematic diagram, a pipette according to theinvention.

FIG. 2 presents, enlarged, the operating means of the pipette of FIG. 1,

FIG. 3 presents, enlarged, the threaded rod of the operating means ofFIGS. 1 and 2, and

FIG. 4 presents the threaded rod according to another embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 is depicted a pipette according to the invention. The pipettecomprises a cylinder 1 with plunger 4, and an operating means 5 withpower means 9. The cylinder 1 constitutes a cylinder volume 2, intowhich leads a liquid passage 3 formed by the tip 10. The plunger 4 hasbeen disposed in the cylinder volume 2 to be reciprocatingly movable.The plunger is provided with a packing 11. The operating means 5comprises a body 6, within which a threaded rod 7 is carried to befreely rotatably but substantially immovably relative to the body in theaxial direction of the threaded rod. Furthermore, within the body 6 hasbeen installed an operating member 8 to be freely movable in the axialdirection of the threaded rod 7 and non-rotatably relative to the body.The threaded rod 7 is with the aid of a thread connected to theoperating member, that is, the external thread of the threaded rod isfitted to work with the internal thread of the operating member. Theoperating means 9 further comprises a power means 9, such as an electricmotor, connected through power transfer means, such as gears 12, withthe threaded rod 7. The cylinder 9 is attached in extension of the body6 to be substantially immovable, and the plunger 4 is attached inextension of the threaded rod to be mainly immovable, so that theplunger can be moved to and fro in the cylinder volume along with thethreaded rod when the threaded rod 7 and the operating member 8 are setin rotary motion relative to each other with the aid of the power means9. The running direction of the power means, and thus of the threadedrod and along with it the plunger, is regulated with the aid of pushbuttons 13, in order to fill or discharge the pipette. The generaldesign of the pipette depicted in FIG. 1 is substantially known in theart and is therefore not described more closely in this context.

As taught by the present invention, the pitch of the thread on thethreaded rod 7 is non-linear, locally in a certain transition range a,see FIGS. 2-3. In the embodiment here presented, the thread isnon-linear in the way that uniform rotary motion of the threaded rod andthe operating member relative to each other, transmitted by the thread,causes a maximum in the axial, uniform movement relative to each otherof the threaded rod and the operating member in that range whichcorresponds to the start of the plunger movement from zero volume of thecylinder in the filling direction. In the present embodiment,non-linearity of the thread has been accomplished by means of a specialdesign of the thread. The flank angle of the thread flank is constant,α, on part of the thread (the portion in the direction of arrow A), andit is constant and larger, α¹, on another part of the thread (theportion in the direction of arrow B). The smaller flank angle αincreases up to the larger flank angle α¹ in the transition range a,which corresponds just to the desired point of non-uniform plungermovement, that is to the maximum when the movement of the plunger iscommenced, from zero volume of the cylinder in the filling direction.The change of flank angle of the thread just described concerns, in theembodiment presented, only that flank of the thread, that is thesupporting surface of the thread, on which the inner thread of theoperating member rests, i.e., the angle of the flank opposite to theflank of the thread relative to the plunger, or in the figure the upperflank, against the normal on the axis of the threaded rod. The flankangle of the opposite flank of the threaded rod is constant throughout,or α¹, in the embodiment presented. Thus, the pitch of the thread ismainly constant and uniform.

In the embodiment depicted in FIG. 3, the transition range of the threadis equivalent to rotation of the threaded rod 7 and the operating member8 relative to each other through 180°. If desired, the transition rangemay equally be wider or narrower, depending on the dimensioning of thecylinder in other respects and on the error between the cylinder's trueand theoretical filling volumes which one desires to compensate with theaid of the transition range.

In the embodiment depicted in FIG. 4, the pitch is larger on a portion(C) of the thread and smaller on the remainder (D) of the thread. Thetransition range a between these portions corresponds to the transitionrange of the linear thread, described in the foregoing.

We claim:
 1. A pipette, comprising: a cylinder constituting a cylindervolume with a liquid passage; a plunger placed to be reciprocatinglymovable in said cylinder volume; and a plunger operating means formoving the plunger in the cylinder; said operating means comprising abody, a threaded rod, an operating member cooperating by mediation ofthreads with said threaded rod, and a power means for rotating thethreaded rod and the operating member relative to each other; said bodybeing connectable to the cylinder and the threaded rod/operating membercombination, to the plunger, for moving the plunger by mediation of thethreaded rod and the operating member when the threaded rod and theoperating member are rotated relative to each other with the aid of thepower means, characterized in that the thread of the threaded rod oralternatively that of the operating member, is non-linear locally over acertain transition range; and characterized in that the thread isnon-linear in such a manner that uniform rotary motion of the threadedrod and the operating member relative to each other, mediated by thethread, causes a maximum of the axial movement of the threaded rod andthe operating member in that range or the thread which corresponds tostarting the plunger movement from zero volume of the cylinder volume inthe filling direction.
 2. Pipette according to claim 1, characterized inthat the pitch of the thread is mainly uniform.
 3. Pipette according toclaim 2, characterized in that the flank angle of one flank of thethread is constant (α) on a portion of the thread and constant andlarger (α¹) on another portion of the thread, and the smaller flankangle (α) increases up to the larger flank angle (α¹) in a transitionrange.
 4. Pipette according to claim 3, characterized in that on part ofthe thread there is a uniform pitch and on the remaining part, a pitchdiffering from the preceding.
 5. Pipette according to claim 4,characterized in that the transition range is equivalent to rotationthrough less than 360° of the threaded rod and the operating memberrelative to each other.
 6. Pipette according to claim 2, characterizedin that the flank angle of one flank of thread is constant on a portionof the thread, and the flank angle on another portion of the thread iseither larger or smaller than the constant flank angle of one flank ofthread, when the larger flank angle is constant, the smaller flank angleincreases up to the larger flank angle in a transition range, and whenthe smaller flank angle is constant, the larger flank angle decreasesdown to the smaller flank angle in a transition range.
 7. Pipetteaccording to claim 6, characterized in that on part of the thread thereis a uniform pitch and on the remaining part, a pitch differing from thepreceding.
 8. Pipette according to claim 7, characterized in that thetransition range is equivalent to rotation through less than 360° of thethreaded rod and the operating member relative to each other.